A group of London residents say the police are not responding quickly enough to their calls, so they have set up a vigilante group. Kick-start a discussion on when community action becomes rule by lawless mobs.

Some concerned householders in Highgate, north London, are unhappy with the way the police react when there is a burglary. They have begun to patrol the streets themselves, as one resident put it, "to see if we can scare these people off". They will not break the law, but will use force to protect themselves.

Vigilante is a word to describe people who band together informally to protect their interests. The word, and the idea, come from the US, where there is a tradition of self-help and a strong emphasis on people's right to protect themselves and their property. Talk to young people about vigilante action they may have come across - perhaps in films. Talk about the emotions it provokes.

For some people, "taking the law into your own hands" is an attractive idea. It appeals to natural justice, especially when the workings of the law seem slow or remote. Do young people agree? Or do they see dangers in this kind of action?

Vigilantes usually say they will act within the law. But it might not seem that way to those on the receiving end of their actions. It is not legal to force people from their homes, as happened to people suspected of offences against children following a newspaper campaign against paedophiles a few years ago.

How do young people assess the risk of mistaken identity? In one well-publicised action, a paediatrician - a doctor specialising in children's illness - was targeted by vigilantes acting against paedophiles. They had got their words mixed up.

In a more recent case, a family was terrified when a rumour spread on the internet that they were harbouring Maxine Carr, the one-time girlfriend of convicted murderer Ian Huntley. They were under nightly siege by a mob who threatened to burn their house down. Is it possible to draw a line between responsible self-protection and mob rule?